Cements of various types have been used in construction for thousands of years, including in concrete. Conventional cements set in the presence of water.
Marcus Vitruvius Pollio, a Roman architect and engineer in the 1st century BCE wrote his “Ten books of Architecture”. (Vitruvius, “The Ten Books of Architecture,” Harvard University Press, 1914) Writing about concrete floors, for example, he said:
“First I shall begin with the concrete flooring, which is the most important of the polished finishings, observing that great pains and the utmost precaution must be taken to ensure its durability”. (page 202)
“On this, lay the nucleus, consisting of pounded tile mixed with lime in the proportions of three parts to one, and forming a layer not less than six digits thick.” (page 203)
The Romans also knew of cement that could set under water.
“There is also a kind of powder [POZZOLANA] from which natural causes produces astonishing results. This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers are constructed of it in the sea, they set hard under water.” (Vitruvius, pages 46-47.)
Today, ordinary Portland cement, OPC is synthesized in cement kilns at high temperatures about 1450° C. During this production, about 1 ton of CO2 emitted per ton of OPC produced. There have been several attempts to decrease the CO2 emissions. The first approach is to increase the efficiency of the cement kilns. Today, the most efficient cement kiln can reduce the CO2 emissions from 1 ton to 816 kg per ton of OPC. The second approach is to blend the cement clinker with supplementary cementitious materials, SCM, which are mainly fly ash, slag and sometimes with burnt oil shale. These SCM's are by products of other processes. However, the major problem with SCM is the variation from source to source, abundance, and geographical dependence of the sources.
Concrete is the second most utilized material on earth after water. Concrete is made of cement, water sand and aggregate. Among the concrete components, ordinary Portland cement (OPC) is produced in high temperature kilns and during cement production about 1 ton of CO2 is emitted per ton of cement produced. Due to high volume of cement production worldwide, the cement industry is responsible for 5% of global CO2 emissions. There have been attempts on reducing the CO2 emission in the cement industry.
Cement is typically made of limestone and shale to form the major phases of alite (C3S in cement chemist notation, Ca3SiO5, sometimes formulated as 3CaO.SiO2) and belite (C2S in cement chemist notation, Ca2SiO4, sometimes formulated as 2CaO.SiO2). Both alite and belite are rich in calcium and they hydrate in the presence of water.
There is a need for sustainable cements that can be manufactured with low CO2 emissions in the existing cement plants with the existing raw material with a reliable and sustainable supply chain, and that consume CO2 as a reagent.